Why Plants Turn Red
Red coloration in aquatic plants comes from anthocyanin pigments produced as a response to high light intensity. Plants produce anthocyanin to protect chlorophyll from photodamage.
No high light = no red. Even with perfect CO2 and ferts, low-tech tanks produce green or pale-pink versions of "red" plants.
Light Requirements
60+ PAR at substrate is the minimum for true red coloration.
For deep red: 80–120 PAR. Recommended fixtures: ADA Solar RGB, Twinstar SA, Chihiros Vivid II.
Spectrum matters: full-spectrum or red-heavy lights enhance red. Cool white (10000K+) suppresses red.
CO2 Requirement
30 ppm CO2 is non-negotiable. Without it, plants pale green and stunt regardless of light or nutrients.
Lean CO2 = brown leaves and BBA on the plant tips.
Iron and Micronutrients
Iron is the headline nutrient for red plants but is rarely the bottleneck. Standard EI dosing or 0.5–1.0 ppm Fe weekly is sufficient.
More important: full trace metal balance (Mn, Zn, Cu, B). CSM+B or Tropica Specialised provides this.
Top Red Species
Rotala rotundifolia (red form): the easiest red. Pinkish-red under high light. Stem plant, fast growing.
Ludwigia repens: deep red-brown. Stem plant, easy to grow.
Ludwigia super red: brilliant red, smaller leaves than repens.
Alternanthera reineckii: dramatic red-purple, broad leaves. Demanding — high light + CO2 + iron.
Rotala macrandra: extreme red, red-pink. Most demanding red plant.
Rotala wallichii: needle leaves, reddish-pink tops. Demanding.
Why Red Plants Stay Green
Light too weak: most common problem. Increase or get a higher-PAR fixture.
CO2 too low: check drop checker is yellow-green during photoperiod.
Photoperiod too short: reds need 7+ hours of intense light.
Plants too new: takes 2–4 weeks to fully color after transitioning to ideal conditions.
Substrate competition: stem plants in shade of foreground plants don't get enough light.